You might ask his doctors about a possible fungal infection. They can become systemic.
why did they give him antibiotics?
the toe nail fungus might be only coincidental or it flared up because his immune system is challenged by something else, but probably not the origin of the problem.
he can try and treat it with Baking Soda. it's very effective against mold.
other effective home remedies are vinegar or bleach.
does he have any other symptoms at all? did he have cold symptoms, loose stool, UTI?
his cognitive functions are normal? does he look like he's been slowed down a bit? loss of short term memory, back pain, clumsy movements?